US President Barack Obama said he is "surprised and troubled" by Republican lawmakers he says are echoing their presidential nominee's positions on Russia.

President Barack Obama on Tuesday (Wednesday NZ Time) accused Republican Donald Trump of showering praise and modelling his policies on Russian President Vladimir Putin to a degree that is "unprecedented in American politics."

Obama said he is "surprised and troubled" by Republican lawmakers he says are echoing their presidential nominee's positions on Russia. Trump has praised Putin as a strong leader and criticised Obama and the Democratic presidential nominee, Hillary Clinton, for Washington's deteriorating relationship with Moscow.

In an interview on Monday (Tuesday NZT), Trump said Russia "can't stand" either Democrat. Trump promised a closer relationship with Putin, if elected, starting with a possible meeting with Putin before the US inauguration.

JONATHAN ERNST/REUTERS

Republican presidential nominee Donald Trump termed the email fallout surrounding rival Hillary Clinton after the Benghazi attack as "one of the great miscarriages of justice".

Obama responded from the White House, where he is hosting Italy's Prime Minister Matteo Renzi for a state visit. The remarks come as Trump and his Republican allies are looking for ways to swing momentum their way after a damaging few weeks in the campaign.

On Tuesday (Wednesday NZT), Republicans seized the latest developments in the Clinton email controversy as an issue that could help Trump make up ground in the final presidential debate.

It follows news that the State Department had asked FBI officials to lower the classification of a sensitive email related to the attack on the US compound in Benghazi, Libya. FBI notes released on Monday (Tuesday NZT) revealed discussion of a "quid pro quo" in trying to get the email reclassified, though it's not clear who first raised the issue and both State and FBI officials deny any bargaining took place.

Obama said on Tuesday (Wednesday NZT) the version Trump and other Republicans are talking about is "just not true."

Trump called it "one of the great miscarriages of justice" in history. But with Wednesday's (Thursday NZT) debate approaching, Trump and his campaign have had trouble sticking to the message.

The candidate has continued his warnings that the election is "rigged," angering not only Democrats but fellow Republicans who worry his rhetoric will hurt public faith in elections. And as news about the emails hit, Melania Trump made her first public comments about the allegations of sexual assault and misconduct lodged against her husband.

In an interview with Fox News aired on Tuesday (Wednesday NZT), Melania Trump said she believes the accusations were coordinated by political rivals: "They want to damage the presidency of my husband, and it was all planned, it was all organised from the opposition."

Trump's comments carried echoes of Clinton's allegations of a "vast right-wing conspiracy" organised to raise similar allegations against her husband two decades ago. Trump notably tried to revive Bill Clinton's history by inviting his accusers to the last debate. His guest list for Wednesday's (Thursday NZT) faceoff in Las Vegas signalled he hoped to change the subject.

The Trump campaign said on Tuesday (Wednesday NZT) that Pat Smith, whose son, Sean Smith, was killed in the attack in Benghazi, will be attending the debate in Las Vegas as the candidate's guest.

Smith was a featured speaker at the Republican National Convention, where she delivered an emotional speech blaming Clinton for her son's death and accusing her of lying to families about what sparked it.

Clinton planned to spend Tuesday (Wednesday NZT) in New York preparing for the debate in Las Vegas. Trump was slated to hold rallies in Colorado.

The disclosure of FBI documents revives questions about Clinton's use of personal email during her time as secretary of state. The issue that has dogged her campaign and damaged voters' trust in her even as she remains the favourite ahead of the November 8 vote.

The document released on Monday (Tuesday NZT) show a State Department Undersecretary for Management Patrick F Kennedy, a former close Clinton aide, contacted an FBI official seeking to change an email's classification, a move that would have sent it to the archive, out of public view. Notes on the conversation describe discussion about a "quid pro quo" in which the email's classification would be changed and State would allow the FBI to place more agents in countries where they hadn't been permitted.

The FBI's records of the interviews indicate that Kennedy made that suggestion, but both the FBI and State Department said on Monday (Tuesday NZT) that it was the unidentified FBI official who first raised the idea of a quid pro quo. Neither the declassification nor the increase in agents occurred. Clinton campaign allies argued on Tuesday (Wednesday NZT) that the documents simply show bureaucratic haggling.

"None of this has anything to do with Hillary Clinton. She wasn't involved in this at all," said Rep Jerry Nadler, a Democrat from New York and a Clinton ally. Nadler said "maybe" the conduct of the officials involved "should be looked at."

The news came as Clinton is trying to expand her edge over Trump - even in Republican territory. Her campaign announced on Monday (Tuesday NZT) it was launching a new push in Arizona, including a campaign stop in Phoenix by first lady Michelle Obama, one of Clinton's most effective surrogates.

An additional US$1 million is going into efforts in Missouri and Indiana, both states with competitive Senate races, and a small amount of TV time is being bought in Texas and media appearances are scheduled in Utah.

On the other side, Trump's campaign dramatically expanded its ad buys in seven battleground states and announced plans to launch a US$2 million advertising blitz in long-shot Virginia.